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Abstract BackgroundLife cycle evolution includes ecological transitions and shifts in the timing of somatic and reproductive development (heterochrony). However, heterochronic changes can be tissue‐specific, ultimately leading to the differential diversification of traits. Salamanders exhibit alternative life cycle polymorphisms involving either an aquatic to terrestrial metamorphosis (biphasic) or retention of aquatic larval traits into adulthood (paedomorphic). In this study, we used gene expression and histology to evaluate how life cycle evolution impacts temporal reproductive patterns in males of a polymorphic salamander. ResultsWe found that heterochrony shifts the distribution of androgen signaling in the integument, which is correlated with significant differences in seasonal reproductive gland development and pheromone gene expression. In the testes, androgen receptor (ar) expression does not significantly vary between morphs or across seasons. We found significant differences in the onset of spermatogenesis, but by peak breeding season the testes were the same with respect to both histology and gene expression. ConclusionThis study provides an example of how seasonal heterochronic shifts in tissue‐specificargene expression can disparately impact seasonal development and expression patterns across tissues, providing a potential mechanism for differential diversification of reproductive traits. 

Abstract Observations on the ontogeny and diversity of salamanders provided some of the earliest evidence that shifts in developmental trajectories have made a substantial contribution to the evolution of animal forms. Since the dawn of evo‐devo there have been major advances in understanding developmental mechanisms, phylogenetic relationships, evolutionary models, and an appreciation for the impact of ecology on patterns of development (eco‐evo‐devo). Molecular phylogenetic analyses have converged on strong support for the majority of branches in the Salamander Tree of Life, which includes 764 described species. Ancestral reconstructions reveal repeated transitions between life cycle modes and ecologies. The salamander fossil record is scant, but key Mesozoic species support the antiquity of life cycle transitions in some families. Colonization of diverse habitats has promoted phenotypic diversification and sometimes convergence when similar environments have been independently invaded. However, unrelated lineages may follow different developmental pathways to arrive at convergent phenotypes. This article summarizes ecological and endocrine‐based causes of life cycle transitions in salamanders, as well as consequences to body size, genome size, and skeletal structure. Salamanders offer a rich source of comparisons for understanding how the evolution of developmental patterns has led to phenotypic diversification following shifts to new adaptive zones.